The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. It has now been demonstrated that it is advantageous to have full access, or substantially full access, to a well casing during a well stimulation treatment. Full access to the casing permits use of downhole tools which are often required, or at least very advantageously used, during a stimulation treatment. An apparatus for providing full access to the casing while permitting stimulation treatments at extreme pressures that approach a burst pressure rating of the casing is described in applicant's U.S. Pat. No. 5,819,851 which issued on Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH PRESSURE OIL/GAS WELL STIMULATION. The patent describes an apparatus for protecting blowout preventers during well fracturing and/or stimulation treatments. The apparatus includes a hollow spool that has spaced-apart inner and outer sidewalls that define an annular cavity. The mandrel is forcibly reciprocatable in the cavity. The mandrel includes an annular seal at a bottom end for sealingly engaging a bit guide attached to the top end of the casing. The apparatus is mounted above a blowout preventer (BOP) attached to a casing spool of the well before well stimulation procedures are begun. The mandrel is stroke down through the BOP to protect it from exposure to fluid pressure as well as abrasive and/or corrosive well stimulation fluids, especially extreme pressures and abrasive proppants. The BOP protector provides a simple, easy to operate apparatus for protecting BOPs which provides full access to the well casing with well servicing tools to facilitate well stimulation at pressures approaching the burst pressure rating of the well casing.
The BOP protector has been readily accepted by the industry and has proven to be an effective tool which reduces the cost of well stimulation treatments while enabling an ultimate choice in treatment options. Field experience has shown that the bit guides of used wellheads tend to become deformed by small chips, dents or scratches after a period of running in and out with production tubing and/or downhole tools. Consequently, the annular seal described in U.S. Pat. No. 5,819,851 sometimes permits pressure leakage under high stimulation pressures.
There therefore exists a need for an improved seal structure for sealing against a bit guide in a well bore which ensures a leak-proof seal at elevated stimulation pressures.
It is therefore an object of the invention to provide an improved high pressure fluid seal for sealing against a bit guide in a wellhead.
It is a further object of the invention to provide a high pressure fluid seal for sealing against a bit guide in a wellhead which maintains an impervious seal when exposed to very high stimulation pressures.